Metallic finish of automobiles is generally carried out as follow: A steel panel which is pretreated with zinc phosphate etc. is primered by electrocoating and cured. Then, the primered panel is intermediated coated and cured. The intermediated coated panel is coated with a thermosetting metallic paint and then, without baking it, coated with a thermosetting clear paint. The coating process is called "Wet On Wet" coating. The wet on wet coated panel is subsequently baked to cure both metallic and clear coatings. The process is also called "2 coat 1 bake".
The metallic paint for the "Wet On Wet" coating process conventionally employs an organic solvent as a diluent, because it has to be rapidly dried and kept surface smooth so as to coat thereon the clear paint without roughing the interface between the metallic and clear coatings. On the other hand, the use of such organic solvent borne paint is apt to be limited in view of environmental pollution, working atmosphere and natural resouces saving. Accordingly, water borne paint becomes more noteworthy. However, when water is employed as a diluent of metallic paint, there are two big problems. One is that, when metallic pigment is incorporated into water borne paint, the metallic pigment is reacted with water to generate hydrogen gas which may be often associated with explosion danger. This problem is especially serious in automotive industries in which paint is stored in a closed container for a long period of time before use.
The second problem is that, when water is employed as a diluent, it is very difficult to control an evaporation loss of water when coating, and therefore impossible to obtain good appearance. For example, in case where coating is carried out at a high relative humidity, an evaporation loss of water is very small and the viscosity of the paint remains low. It may cause sagging and poor orientation of metallic pigment, which provides poor appearance. In case where coating is carried out at a low relative humidity, an evaporation lass of water is large and the viscosity of the paint is too high. It may cause the decline of flowability of the paint, which also provides poor appearance.